The disposal of used and discarded tires has posed a problem for almost 100 years, since transportation first began to use rubber tires. Disposal of waste tires is a dilemma for cities, states and industries throughout the United States. It is estimated that at the present time, from two to five billion waste tires occupy landfills and stockpiles. Approximately 280 million additional waste tires are generated each year in the United States alone. For many years governmental agencies, municipalities, industrial corporations and community leaders have worked to reduce this growing stockpile of discarded tires.
Waste tires pose an environmental and health problem. They are not biodegradable. They are unsightly, and provide insect breeding grounds, thereby exacerbating the spread of mosquito-borne diseases. Also, whole tires are difficult to store in landfills because they tend to work their way to the top of the fill and can cause damage to the landfill cap or seal. Many tires are dumped illegally because of the cost associated with placing them in sanctioned dump sites.
In desperation, almost all states have enacted laws, proposed legislation, or adopted proposals to regulate the collection and disposal of waste tires. Already many of these states collect license fees or taxes which are routed to industry and universities for small business start up incentives. A number of states provide R&D funding to solve the waste tire disposal problem.
This has led to investment in tire shredders and similar disposal technologies, so that tires will take up less volume when stored in landfills, and to an increased demand for an efficient and cost-effective method of recycling the tires so that landfilling can be avoided.
The difficulty in processing waste tires arises in separating each tire into its multiple components. The body of a tire typically comprises rubber tread and rubber coated sidewalls. Both natural and synthetic rubbers are used in tire manufacturing, including natural rubber, styrene butadiene (SBR) rubber, polybutadiene rubber, butyl rubber and polyisoprene rubber. To obtain superior strength and wear resistance, the rubber is vulcanized. The resulting toughness contributes to the indestructibility of tires.
In addition, tires are typically reinforced with belts of steel wire and reinforcing fabric or fiber, and include a metal beading strip around their inner diameters. These components are wrapped and bonded together by rubber layers to form a tough, durable tire. It is this very toughness, however, which impedes recycling of the materials in the tire.
There is a commercial market for products, such as scrap rubber, steel, and polyester fiber, recovered from waste tire recycling, provided the cost of the recovery is relatively low. For example, recycled rubber from waste tires can be used in new tires, in asphalt pavement, and in industrial materials.
Most techniques used to recover the recyclable components of waste tires require some initial physical processing, followed by further refining by some secondary process. For example, initial physical processing steps may include: debeading, cryogenic crushing, separation of the sidewalls from the tread,, shredding, and washing.
In cryogenic crushing, liquid nitrogen is used to lower the temperature of the tire to about -300.degree. F. At such a low temperature, rubber hardens and fractures into tiny granules leaving the steel and cotton fibers relatively unaffected. The steel is removed by electromagnets and the cotton is removed by sieving. The cost of cryogenic processing, however, is relatively high.
Secondary processing methods, which convert the materials into a reusable form, include: mechanical reclamation, chemical digestion, destructive distillation, and hydrogenation. In mechanical reclamation, a "reclamator" is used to remove the rubber tread from the fiber and steel reinforcements solely by mechanical means. The reclamator is a type of screw extruder generating its own heat by the mechanical working of the rubber crumb.
In chemical digestion, tires are ground up in a mill and the ground particles are digested in an autoclave at temperatures of approximately 200.degree. C. and a pressure of approximately 400 psi for 5 to 25 hours, in various acids, alkalies or salts mixed with softening oils. At the completion of digestion, the fabric has been dissolved, such as by acids, and the rubber is fragmented into molecules which can be reworked.
Various other chemical processes have been used to recover the chemical constituents of scrap tires. For example, pyrolysis is used to achieve destructive distillation and carbon black recovery. Hydrogenation is used to add hydrogen to the rubber to change its chemical composition to form products from which new elastomers can be produced. Such chemical processes are costly and therefore are of limited value. Also, depending on the nature of the chemical treatment, the rubber may become devulcanized. In some uses for reclaimed rubber devulcanized rubber is preferred, while other recycle applications work as well or better with rubber that has not been devulcanized. Applications in which it is preferred or possible to use vulcanized rubber include asphalt rubber binders for use in asphalt pavement, in carpet underlayers, and tire retreading.